The present invention relates to an image forming apparatus in a copying machine, a printer, a facsimile, a compound machine of those or the like.
In conventional image forming apparatus of electrographic type, a paper sheet having an unfixed toner image formed on a surface thereof is passed through a fixing device, and the toner image is thereby heated and fused to be fixed on the sheet, which is thereafter ejected from the image forming apparatus and is stacked and stored on an ejected-paper tray.
When the sheet is passed through the fixing device, curl is given to the sheet by influence of heat transferred to the sheet. An increase in amount of the curl deteriorates condition of the stacking, when a large number of sheets are stacked and stored on the ejected-paper tray, and thus may make it impossible to store a number of sheets that can be stacked and stored on condition that the amount of the curl is small.
It has been known that an amount of curl of a paper sheet can be reduced by decrease in fixation temperature on occasion of thermal fixation for the sheet. In an image forming apparatus disclosed in Japanese Laying-Open Publication No. 2000-162920, for solution of the problem of deterioration in stacking ability, a fixation temperature is controlled to be set lower than normal one when a number of ejected sheets reaches a specified number such that influence of the curl on sheet stacking ability on an ejected-paper tray appears.
The decrease in the fixation temperature of a fixing device as in the image forming apparatus disclosed in the publication, however, causes another problem of deterioration in image fixation performance.